The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.
The present invention relates to thrusters, and, in particular, relates to thrusters for controlling spacecraft.
A key technology to lowering the cost of launching and maintaining future satellites are new, efficient propulsion systems. Electric propulsion thrusters are of great interest because of their substantially higher exhaust velocity compared with traditional chemical propulsion thrusters which allows for significant mass reduction of the spacecraft propulsion system, thereby increasing the payload to spacecraft mass ratio. Highly promising thruster designs already finding use are ion and Hall-effect thrusters. In these engines, a gas is efficiently ionized and electrostatically accelerated to provide thrust. The current gas of choice has been xenon, given its high mass, relatively low ionization potential, low chemical reactivity and excellent discharge properties. Xenon, however, is very expensive, and it is anticipated that with the growing use of xenon in space, the price will steadily increase during the coming years. There is, therefore, considerable interest in finding cheaper alternative propellants that still meet the required performance criteria. Other noble gases, such as krypton and argon, have been tried, but they don""t have the desired performance that xenon offers given their lower mass and higher ionization potentials. While earlier ion and Hall-effect thruster models included metallic propellants, such as cesium and mercury which met the high atomic mass, low ionization potential requirement, these fuels have many disqualifying drawbacks such as the necessity to heat the metal to generate sufficient vapor pressure, the possibility of depositing metal coatings on insulators and causing short circuits, and environmental concerns at ground level.
Thus, there exists a need for a cost effective thruster that overcomes the above prior art shortcomings.
Broadly the present invention provides an improved spacecraft thruster, either Hall effect or ion, using gaseous propellant evaporated from solid iodine. The means for converting the solid iodine is
a) a tank for iodine crystals,
b) means to control the temperature in the tank,
c) a thrust chamber,
d) a feed tube connecting the tank and the chamber,
e) a filter mounted at the input end of the feed tube proximate the tank,
f) means to control the temperature in the filter,
g) a mass flow controller having a valve for flow control and shut-off mounted in the feed tube between the tank and the chamber and
h) means to control the temperature in the mass flow controller.
While 1 kg of iodine (99.999%) costs approximately $400, the current cost of one kg of xenon (99.995%) is xcx9c$4,000. However, iodine exhibits many desired propellant features: The iodine atomic weight is 126.9 amu versus that of xenon, 131.3 amu. The ionization potential of atomic and molecular iodine are 10.45 eV and 9.4 eV, respectively, versus 12.13 eV of xenon. Since iodine is a solid with sufficient vapor pressure (0.3 Torr at 25xc2x0 C. and 1 Torr at 40xc2x0 C.), considerable mass and volume savings are possible with respect to propellant storage. Potential drawbacks of iodine are the molecular form (versus the atomic form of xenon), its corrosiveness, and its ability to attach electrons.
Therefore, one object of the present invention is to provide a spacecraft thruster fuel which is substantially less expensive than present fuels.
Another object of the present invention is to provide a fuel which does not require a pressurized tank and therefore to reduce the mass of the fuel handling system.
Another object of the present invention is to provide a fuel which either exceeds or meets the efficiency of present fuels such as xenon.
These and other objects and advantages of the present invention will be readily apparent to one skilled in the pertinent art from the following detailed description of a preferred embodiment of the invention and the related drawings.